Adding Subpanel for basement remodel

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Old 02-23-16, 09:24 AM
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Question Adding Subpanel for basement remodel

Main part of house has an ungrounded system. I will be installing a grounding rod to run the ground wire into the subpanel so that all my new circuits will be properly grounded. Is it necessary to install a ground lug in the main panel before running the #4 cable into the subpanel?
 
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Old 02-23-16, 09:32 AM
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Ground has to be bonded to neutral in the panel containing the main disconnect. Assuming your main disconnect is in that main panel then the ground from the rods needs to go in there and connect to the neutral bus. Then you run 3 conductor plus ground to the subpanel and keep ground and neutral separated in the subpanel. That usually means adding a ground buss and removing the neutral bonding screw or jumper in the sub. Both the ground wire and the neutral wire from the cable going to the sub connect to the neutral buss in the main.
 
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Old 02-26-16, 08:23 AM
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Thanks for this information. Do you suggest 3-3-3-5 copper cable to feed the subpanel? I have a 70 amp breaker in the main box for the subpanel.
 
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Old 02-26-16, 08:36 AM
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A ground rod has absolutely no purpose in ensuring the new circuits are grounded. The new circuits are grounded by the grounding conductor run in the new cables.
 
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Old 02-26-16, 08:54 AM
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If you go copper, you'll probably only find 2-2-2-8 copper cable, either NM or SER. Perhaps you'll find 2-2-4-8. Both of these can be used up to 100A. A 70A feed is not a common size panel, I wouldn't expect most suppliers to have that size in stock. If you wanted to drop your breaker down to 60A, you could use #6-3/g cable which is a very common size.
 
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Old 02-26-16, 09:15 AM
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The panel just needs to be rated above the breaker size feeding it.
 
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Old 02-26-16, 09:37 AM
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If it's a long run from Main to Sub you may want to consider aluminum SER. It's quite a bit cheaper than copper. Also be aware that SER has a different temp rating based on where installed than does NM. NM is limited to 60 deg.C when determining amp capacity vs. SER is 75 deg. C as long as it's not buried within insulation. Your 70A breaker is fine for feeding a 100A or 125A sub-panel or larger. Obviously you are limited to 70A load.
 
 

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